﻿Liquid crystal (LC) technology has a major impact on many areas of science and engineering.
It is a promising material for planar lightwave circuit (PLC) devices in optical fiber
communication systems due to its mature and inexpensive technology. It can provide low
power consumption, and low drive voltage because of its attractive anisotropic optical
properties and large electro-optic response.
However, it is also very difficult to deposit the LC film and to pattern the film using typical
photolithography for fabricating PLC based waveguide devices. Therefore, up to now, only a
few number of LC based waveguide devices were proposed. They were fabricated on typical
inorganic materials, rather than polymer ones. Moreover, the major shortcomings in the LC
based integrated optics are excessive insertion loss and relatively high operating voltage. This
project will target at the use of LC in planar optical waveguide devices to replace the typical
thermo-optic effect; and to solve the issues in the fabrication of LC based device by novel
design. There is therefore a strong need for doing research on the design and fabrication in
order to evaluate the characteristics of the LC based electro-optic waveguide device.
In this study, we apply LC in different ways to demonstrate integrated variable optical attenuators (VOA), which is widely used for monitoring and active controlling of optical
channel power in modern high-speed optical wavelength division multiplexed (WDM)
networks. LC is used to change the optical confinement of a waveguide core based on its
electro-optic effect. We initially demonstrate a VOA adopting the LCD technologies and
applying LC’s anisotropic property to fabricate a waveguide using LC as both core and
cladding. A 20dB extinction ratio can be achieved by applying a 10Vpeak at 1550nm. We
further improve the design by employing LC as an active cladding layer in order to minimize
the propagation loss due to LCs’ optical absorption and scattering. We demonstrate the
devices on polymer materials rather than on inorganic materials because polymers are very
attractive for optical devices due to their advantages, such as, simple fabrication processes for
a large variety of geometries, easy formation of multi-layer structure by suitable
micro-fabrication technologies (for example, embossing or UV techniques) and low material
cost for high volume fabrication using embossing or UV techniques. The single mode
polymer inverted channel waveguide having a nematic LC upper cladding operates at a
wavelength of 1550nm. The novel inverted structure is proposed here to provide more
homogeneous LC molecular alignment, since the structure can eliminate the LC alignment
defects at the edge of a normal rib structure. Besides, it protects the channel from damage
during the preparation of the alignment layer by mechanical rubbing. The fabricated device
exhibits a 24dB of attenuation range with a tuning range of 10 Vpeak at 1550nm. This device’s asymmetric attenuation characteristics of the two polarization states enable a 16dB dynamic
range of polarization dependant loss compensation.
Furthermore, we have simulated the device with different core sizes to reduce the controlling
applied voltage. The theoretical results are also verified with experiments. The device’s
response time is measured by applying the modulated signal. Finally, lithography technique is
proposed in making the alignment layer to align with the LC that can potentially replace the
traditional rubbing one for improved performance. This study can also be used as a guide in
design and fabrication of other LC based waveguide devices.